The present invention relates to a semiconductor light emitting device for use in optical transmission, particularly in IEEE (Institute of Electrical and Electronics Engineers) 1394, and in display or the like.
In recent years, semiconductor light emitting devices have being broadly applied on such fields as optical communication and information display panels. The semiconductor light emitting devices for use in these applications are required to have high luminous efficiency. In particular, it is important for the semiconductor light emitting devices for use in optical communication to have high response speed.
Recently, a POF (Plastic Optical Fiber) has been started to be used in communication in relatively short distance. As a light source of the POF, there has been developed a surface-emitting rapid-response LED (Light Emitting Diode) having an emission wavelength in the vicinity of 650 nm, which is a low loss wavelength range for the POF. The active layer of this semiconductor light emitting device is made from an AlGaInP (Aluminum Gallium Indium Phosphide) based semiconductor material capable of high efficiency light emission, and has structure of quantum well. As a means to improve light extraction efficiency of the semiconductor light emitting device, a DBR (Distributed Bragg Reflector) is introduced as a multilayer reflecting film with high reflectance placed in between the active layer and a GaAs (Gallium Arsenide) substrate.
FIG. 9 is a view showing an emission spectrum of a semiconductor light emitting device having an active layer provided with the DBR and the quantum well structure, in which the distance between the DBR and the quantum well layer, that is, the distance between the upper surface of the DBR and the lower surface of the quantum well active layer is approximately 1 xcexcm. In FIG. 9, a horizontal axis represents a wavelength of light (nm) while a vertical axis represents relative intensity of light (a.u.: arbitrary unit). When the semiconductor light emitting device emits a ray of light, a ray of light reflected by a DBR on the lower side of the active layer and returned back to the active layer is absorbed little by the active layer and radiated from the semiconductor light emitting device. This is because the active layer having the quantum well structure is extremely small in thickness. Here, an emitted ray of light from the active layer is modulated through interference with a reflected ray of light reflected by the DBR. This changes the spectrum of the ray of light. More particularly, as seen from an emission spectrum configuration in FIG. 9, troughs caused by the interference appear at an interval of approximately 30 to 40 nm in wavelength, generating sub peaks in both sides of a main peak having a wavelength of approximately 665 nm. This indicates that a phase difference between the ray of light reflected by the DBR and the ray of light emitted from the active layer is approximately a multiple of 2xcfx80.
However, in the prior art semiconductor light emitting device, the emission spectrum configuration is considerably changed with variance in a distance between the upper surface of the DBR and the lower surface of the active layer, causing a change in the peak wavelength. More particularly, in the semiconductor light emitting device having the emission spectrum shown in FIG. 9, a few % increase in the distance between the upper surface of the DBR and the lower surface of the active layer forms an emission spectrum configuration as shown in FIG. 10. Compared with the emission spectrum configuration in FIG. 9, the peak in FIG. 9 is replaced with a trough of the emission spectrum configuration in FIG. 10, and the wavelength of the main peak moves to a short wavelength side by approximately 15 nm to be approximately 650 nm. In other words, a slight change in the distance between the upper surface of the DBR and the lower surface of the active layer transforms a peak of interference to a trough, causing a displaced peak wavelength.
In the case where the semiconductor light emitting device is used as a light source for POF communication, the low loss wavelength range of the POF is as small as around 40 nm, so that the peak wavelength in the emission spectrum of the semiconductor light emitting device is required to be set within the low loss wavelength range without displacement. In other words, the distance between the upper surface of the DBR and the lower surface of the active layer should be set with high accuracy. Accordingly, in the process of manufacturing semiconductor light emitting devices, a clad layer and the like placed in between a DBR and a quantum well active layer requires high-accuracy layer thickness control in particular. This leads to a problem of decrease in a yield of the semiconductor light emitting devices.
Accordingly, it is an object of the present invention to provide a semiconductor light emitting device which is not affected by variance in a distance between an upper surface of a DBR and a lower surface of an active layer, and enables stable provision of a specified peak wavelength in an emission spectrum.
To accomplish the above object, a first aspect of the present invention provides a semiconductor light emitting device having in sequence on a semiconductor substrate, a multilayer reflection film, a semiconductor layer, and a quantum well active layer, wherein when a light emission wavelength is xcex (xcexcm), and an average refractive index of the semiconductor layer disposed in between the multilayer reflection film and the quantum well active layer is n, a distance between an upper surface of the multilayer reflection film and a lower surface of the quantum well active layer is 2xcex/n (xcexcm) or less, and a phase difference between a reflected ray of light reflected by the multilayer reflection film and an emitted ray of light from the quantum well active layer is a multiple of 2xcfx80.
According to the first aspect of the semiconductor light emitting device, the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer is set to be 2xcex/n or less, and a phase difference between a reflected ray of light reflected by the multilayer reflection film and an emitted ray of light from the quantum well active layer is set to be a multiple of 2xcfx80. As a result, in the emission spectrum configuration of the semiconductor light emitting device, an interval between troughs generated by interference between the reflected ray of light and the emitted ray of light becomes relatively large. Accordingly, even if the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer is slightly changed and therefore the position of troughs in the emission spectrum configuration is slightly displaced, the troughs will not match peaks. Therefore, with slight variance in the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer, there is almost no difference in a peak wavelength of the semiconductor light emitting device. This enables stable provision of the semiconductor light emitting device having a specified peak wavelength without a necessity of high-accuracy thickness control.
A second aspect of the present invention provides a semiconductor light emitting device having in sequence on a semiconductor substrate, a multilayer reflection film, a semiconductor layer, and a quantum well active layer, wherein when a light emission wavelength is xcex (xcexcm), and an average refractive index of the semiconductor layer disposed in between the multilayer reflection film and the quantum well active layer is n, a distance between an upper surface of the multilayer reflection film and a lower surface of the quantum well active layer is 15xcex/n (xcexcm) or more.
According to the second aspect of the present invention, in the emission spectrum configuration of the semiconductor light emitting device, an interval between troughs generated by interference between the reflected ray of light reflected by the multilayer reflection film and the emitted ray of light from the active layer becomes small. For example, when an average refractive index n of the semiconductor layer equals to 3.0, and an emission wavelength xcex of the semiconductor light emitting device equals to 0.65 xcexcm, the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer is about 3 xcexcm based on 15xcex/n. In this case, as seen from FIG. 8, the interval between troughs generated by interference between a reflected ray of light reflected by the multilayer reflection film and an emitted ray of light from the active layer becomes approximately 15 nm. FIG. 8 is a view showing, in the semiconductor light emitting device in which an average refractive index n of the semiconductor layer in between the multilayer reflection film and the quantum well active layer equals to 3.0, and an emission wavelength xcex equals to 0.65 xcexcm, an interval between the troughs of interference corresponding to a distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer. A horizontal axis represents a distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer (xcexcm), while a vertical axis represents an interval between troughs of interference (nm). As seen from FIG. 8, when the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer equals to 3 xcexcm or more, the interval between troughs of interference becomes less than 15 nm. Consequently, a ray of light emitted from the semiconductor light emitting device has at least one peak wavelength in a wavelength range of 16 nm. Therefore, the semiconductor light emitting device is suitable as a light source of the POF for IEEE 1394 which requires control of a peak wavelength of a ray of light emitted at a room temperature to be approximately 16 nm or less. Further, with larger distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer, the emission spectrum configuration acquires a number of plural peaks and troughs compared to the case of no interference. However, the configuration of an envelope curve obtained by connecting these plural peaks stays about the same as the configuration of a spectrum without interference. In addition, if the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer is slightly varied, and the number of the plural peaks and troughs in the emission spectrum configuration is changed, or the troughs of interference match peaks in the emission spectrum in the case of no interference, the configuration of an envelope curve in the emission spectrum shows almost no difference from the configuration of the emission spectrum in the case of no interference. In other words, the semiconductor light emitting device is almost free from change in the peak wavelength. This enables stable provision of the semiconductor light emitting device having a specified peak wavelength without a necessity of high-accuracy thickness control.
In one embodiment of the present invention, the semiconductor substrate is composed of GaAs, InP, ZnSe, or GaN.
According to the embodiment, luminous efficiency of the semiconductor light emitting device is increased by forming, on the semiconductor substrate, a semiconductor layer and an active layer made of a semiconductor which is lattice-matched with GaAs, InP, ZnSe, or GaN.
In one embodiment of the present invention, a distance between an upper surface of the multilayer reflection film and a lower surface of the quantum well active layer is 0.4 xcexcm or less.
When the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer is set to be 0.4 xcexcm or less, and the semiconductor layer disposed in between the multilayer reflection film and the quantum well active layer is made from a material that is lattice-matched with a GaAs substrate such as AlyGazIn1xe2x88x92yxe2x88x92zP (Aluminum Indium Phosphide) (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), and AlxGa1xe2x88x92xAs (Aluminum Gallium Arsenide) (0xe2x89xa6xxe2x89xa61), each having a refractive index of 3 to 3.5, a peak wavelength of the semiconductor light emitting device is sustained around 650 nm. This enables stable provision of the semiconductor light emitting device suitable, for example, as a light source of is a POF without a necessity of high-accuracy thickness control.
In one embodiment of the present invention, a distance between an upper surface of the multilayer reflection film and a lower surface of the quantum well active layer is 3 xcexcm or more.
When the distance between the upper surface of the multilayer reflection film and the lower surface of the quantum well active layer is set to be 3 xcexcm or more, and the semiconductor layer disposed in between the multilayer reflection film and the quantum well active layer is made from a material that is lattice-matched with a GaAs substrate such as AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), and AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61), each having a refractive index of 3 to 3.5, a peak wavelength of the semiconductor light emitting device is sustained around 650 nm. This enables stable provision of the semiconductor light emitting device suitable, for example, as a light source of a POF without a necessity of high-accuracy thickness control.
In one embodiment of the present invention, the quantum well active layer having a quantum well active layer is composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
According to the embodiment, the quantum well active layer is composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61). This enables provision of the semiconductor light emitting device having a desired wavelength in the wavelength range of luminescent colors from red to green.
In one embodiment of the present invention, the multilayer reflection film is composed of a material that is lattice-matched with GaAs.
This increases crystallinity of an active layer formed on the multilayer reflection film, and facilitates mirror-finish processing of an interface between the multilayer reflection film and a layer on top thereof, resulting in increased reflectance of the multilayer reflection film. As a result, the semiconductor light emitting device obtains higher output.
In one embodiment of the present invention, the multilayer reflection film is composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
According to the embodiment, Aly GazIn1xe2x88x92yxe2x88x92zP forming the multilayer reflection film is lattice-matched with a GaAs substrate. This increases crystallinity of an active layer formed on the multilayer reflection film, and facilitates mirror-finish processing of an interface between the multilayer reflection film and a layer on top thereof, resulting in increased reflectance of the multilayer reflection film. As a result, the semiconductor light emitting device obtains higher output.
In one embodiment of the present invention, the multilayer reflection film is composed of AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa61)
According to the embodiment of the present invention, AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa61) forming the multilayer reflection film is lattice-matched with a GaAs substrate. This increases crystallinity of an active layer formed on the multilayer reflection film, and facilitates mirror-finish processing of an interface between the multilayer reflection film and a layer on top thereof, resulting in increased reflectance of the multilayer reflection film. As a result, the semiconductor light emitting device obtains higher output. In addition, a multilayer reflection film composed of AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa61) has higher reflectance against rays of light having a wavelength from red to yellow than a multilayer reflection film composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61). This enables provision of the semiconductor light emitting device having high output, which emits rays of light having a color from red to yellow.
In one embodiment of the present invention, the multilayer reflection film is composed of a pair of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61) and AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa61).
According to embodiment of the present invention, AlyGazIn1xe2x88x92yxe2x88x92P (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61) and AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa61) which form the multilayer reflection film are lattice-matched with a GaAs substrate. This increases crystallinity of an active layer formed on the multilayer reflection film, and facilitates mirror-finish processing of an interface between the multilayer reflection film and a layer on top thereof, resulting in increased reflectance of the multilayer reflection film. As a result, the semiconductor light emitting device obtains higher output.
In one embodiment of the present invention, a maximum reflectance of the multilayer reflection film against a ray of light from the quantum well active layer is 80% or more.
This increase output of the semiconductor light emitting device.